1. Field of the Invention
The present invention relates to a pneumatic tire aiming at driving on a punishing road including a rocky stretch and a muddy terrain.
2. Description of the Related Art
In an off-road pneumatic tire aiming at a punishing road driving, it is necessary to take into consideration not only driving on a flat paved road or unpaved road, but also driving on a rough punishing road including a muddy terrain. Accordingly, there has been known a tire structured such that a projection is provided in a buttress portion which is not grounded at a normal driving time on the flat paved road, and a traction can be generated by a shearing resistance of the projection in the case of sinking into the muddy terrain (for example, Japanese Unexamined Patent Publication No. 11-291718). Further, there has been known a fact that an external scratch resistance at a time of driving on the punishing road is improved by such a projection (for example, Japanese Unexamined Patent Publication No. 2004-291936).
Incidentally, there is a case that a rocky stretch such as a rugged mountain or the like is included in addition to the muddy terrain, in a driving course of an off-road race. In the rocky stretch, as shown in FIG. 6, there is a condition that it is necessary to drive by grounding only an end portion of a tread surface Tr, and there is a case that a wheel falls out of a rock R in some cases. Generally, a cross sectional width CW of the tire is larger than a tread width TW, and if the tire falls, an outer wall surface 20a of a side wall portion 20 is scuffed and applied to the rock R so as to be greatly deformed to an inner side in a width direction. At this time, there is a case that a load is concentrated on a specific position of the side wall portion 20 so as to damage.
In Japanese Patent No. 3922978, there is described a pneumatic tire in which a convex protector having an elastic reinforcing member built-in is formed in a tire circumferential direction, thereby protecting a side wall portion. However, the protector is only structured such as to protect the side wall portion from an obstacle coming close mainly along a tire axial direction or a tire circumferential direction, and cannot prevent the side wall portion from being damaged with respect to an external scratch factor (for example, an edge portion of a road surface and an angular portion of a rock surface) coming into contact along a tire diametrical direction at a time when the wheel falls.
In other words, in a case where the above-mentioned tire falls, the external scratch factor coming close along the tire diametrical direction may be received by the protector, however, the protector locally deforms in the tire diametrical direction at the received part, and a strain is concentrated on a root so as to easily generate a cut scratch. Since the cut scratch is generated in a center portion in a tire cross section height in which a thickness is comparatively small, the cut scratch tends to cause a cut puncture, and is directly linked to a shortening of a tire service life. In the punishing road drive, it is often the case that the tire is under an environment that it cannot be exchanged, and it is desired to focus on devising a countermeasure against an instantaneous tire function loss such as the cut puncture.
In Japanese Unexamined Patent Publication Nos. 2000-318410 and 2007-245945, there is described a tire in which a flap-shaped projection piece is provided in an outer wall surface of a side wall portion for preventing a water splash from flying at a time of driving in a rainy weather. Further, in Japanese Unexamined Patent Publication No. 5-96649, there is described a tire in which a convex stripe is provided in the vicinity of a mold split surface for preventing an overflow rubber (a burr) formed in the mold split surface from being generated at a time of manufacturing the tire. However, such a projection piece and convex stripe neither be structured such as to protect the side wall portion from the external scratch factor, nor be capable of improving a damage resistance at a time when the wheel falls as mentioned above.
In Japanese Patent No. 3943359, there is described a pneumatic tire in which a protector protruding from an outer wall surface of a side wall portion is provided with an outer annular portion being annularly continuous in a tire circumferential direction, an inner annular portion being annularly continuous in an inner side in a tire diametrical direction thereof, and a concave portion formed by depressing a surface of the protector while leaving a joint portion joining between these pair of annular portions. However, there has been known that there is great fear of causing a damage of the side wall portion due to the structure of the protector at a time when the wheel falls as mentioned above.
In other words, when the above tire falls, there is a case that an inner annular portion 22 which is closer to the center portion of the tire cross section height than an outer annular portion 21 catches on a rock R, as shown in FIG. 7, and there is a risk that a stress is concentrated on a root of the inner annular portion 22 and a puncture is generated by a cut scratch. Accordingly, it is necessary to avoid arranging an element on which the external scratch factor catches in the tire diametrical direction, in the vicinity of the center portion of the tire cross section height in which a thickness is comparatively small, and it is further necessary to keep the outer wall surface of the side wall portion away from the external scratch factor.
Further, since a concave portion 23 is surrounded by the outer annular portion 21 and the inner annular portion 22, in the tire mentioned above, there is a problem that a muddy soil is hard to be discharged from the concave portion 23 at a time of driving on the muddy terrain. Accordingly, in the muddy terrain, a rotational resistance of the tire is increased by the muddy soil clogged in the concave portion 23, and a traction caused by a joint portion is not achieved due to a flattening of the surface of the protector, so that it is unavoidable to deteriorate a punishing road driving performance.